Electrodeposition as a coating application method involves deposition of a film-forming composition onto a conductive substrate under the influence of an applied electrical potential. Electrodeposition has become increasingly important in the coatings industry because, by comparison with non-electrophoretic coating means, electrodeposition offers increased paint utilization, improved corrosion protection and low environmental contamination.
Initially, electrodeposition was conducted with the workpiece to be coated serving as the anode. This was familiarly referred to as anionic electrodeposition. However, in 1972 cationic electrodeposition was introduced commercially and has continued to gain in popularity. Today, cationic electrodeposition is by far the prevalent method of electrodeposition. For example, a cationic primer coating is applied by electrodeposition to more that 80 percent of all motor vehicles produced throughout the world.
Electrodeposition coatings are typically thermally cured in ovens, many of which are gas-fired ovens. Those cured in gas-fired ovens can be exposed to combustion byproducts during the curing step. It is generally recognized that these byproducts and other volatiles in the oven environment may interact with the coating being cured.
Nitrogen oxides, collectively referred to as NOx, can be formed during the combustion of a hydrocarbon fuel, such as natural gas used to fuel gas-fired ovens. Nitrogen oxides can form as a result of two oxidation mechanisms: (1) reaction of nitrogen in the combustion air with excess oxygen (referred to as thermal NOx) and (2) reaction of nitrogen that is chemically bound in the fuel (referred to as fuel NOx). In addition, minor amounts of NOx are formed through complex interaction of molecular nitrogen with hydrocarbons in the early phase of the flame front (referred to as prompt NOx). The majority of NOx produced by combustion exists as nitric oxide (NO), with lesser amounts of nitrogen dioxide (NO2) present. The quantity of NOx created when a fuel burns depends primarily on temperature, time, and turbulence variables. That is, flame temperature and the residence time of the fuel/air mixture, along with the nitrogen content of the fuel and the quantity of excess air used for combustion determine the NOx levels present in the curing oven atmosphere. By delaying the mixing of fuel and air, low NOx burners can reduce combustion temperatures, minimize initial turbulence, and retard the formation of NOx in the curing oven.
Nitrogen oxides in the curing oven can create an oxidizing environment that in turn can interact with the surface of a coating being cured in such an environment. It has been known that the presence of NOx in a curing oven can influence the color and durability of a coating cured in such an environment. It has now been discovered that nitrogen dioxide affects properties of the cured coating to a much greater extent then nitric oxide. The presence of as little as 2 ppm nitrogen dioxide in a curing environment can cause yellowing of a light-colored coating such as a white coating, and/or decrease the durability of a coating system exposed to UV radiation. For example, a subsequently applied topcoat layer exhibits a greater rate of UV-induced delamination with ultraviolet exposure when the underlying electrodeposited layer is cured in an environment where nitrogen dioxide is present. That is, minimization of nitrogen dioxide levels is desirable in order to maximize the performance of a coating cured in a gas-fired oven, regardless of nitric oxide level.
Direct fired ovens are arranged such that the products of combustion are present within the oven environment. Nitrogen oxides are included in these combustion products. It is in this environment that the greatest effect of these gases are seen on the performance of paints cured in such an oven. Low NOx burners can reduce the overall levels of NOx, typically to around 5 ppm, but nitrogen oxides, specifically nitric oxide and frequently nitrogen dioxide, are still present.
Indirect fired ovens utilize heat exchangers through which the products of combustion pass. The environment within such an oven can be free of NOx. However, variations in oven design and maintenance can allow measurable amounts of NOx into the curing environment.
Many industrial applications utilize electrodepositable acrylic coatings, crosslinked with aliphatic isocyanates. Acrylic coatings are typically more stable to ultraviolet radiation-induced degradation than their epoxy-based counterparts. Such coatings are frequently used in one-coat applications, or in conjunction with a clear topcoat. In such applications, color control is important, in addition to durability.Electrodepositable primer coating compositions, particularly those used in the automotive industry, typically are corrosion-resistant epoxy-based compositions crosslinked with aromatic isocyanates. If exposed to ultraviolet energy, such as sunlight, these compositions can undergo photodegradation. In some applications, a primer-surfacer is spray-applied directly to the cured electrodeposited coating prior to application of one or more topcoats. The primer-surfacer can provide a variety of properties to the coating system, including protection of the electrodeposited coating from photodegradation. Alternatively, one or more topcoats can be applied directly to the cured electrodeposited coating and in such instances, these topcoats typically are formulated such that the topcoat(s) provide sufficient protection to prevent photodegradation of the electrodeposited primer coating. If the topcoat(s) do not provide sufficient protection, photodegradation of the electrodeposited primer coating can result in delamination of the subsequently applied topcoats from the cured electrodeposited primer coatings producing catastrophic failure of the cured coating system.
For example, if one or more topcoats are sufficiently opaque to ultraviolet light transmission, such as by a high concentration of pigment and/or light absorbing compounds (e.g. UVAS), little or no ultraviolet light can penetrate through the topcoat(s) to the electrodeposited primer coating to cause photodegradation. However, if a thin topcoat and/or a topcoat which is not ultraviolet light absorbing is applied to the cured electrodeposited primer coating, ultraviolet light can pass through the topcoat(s) resulting in photodegradation of the cured electrodeposited primer coating. Such a problem is also likely to occur when a topcoat is lightly pigmented with mica or metal flake pigments which tend to allow transmission of ultraviolet light to the previously applied and cured electrodeposited primer coating.
A variety of approaches are known to avoid photodegradation of the cured electrodeposited coatings. As mentioned above, topcoats can be formulated to have a high concentration of pigments which provide ultraviolet light opacity. Further, topcoat formulations can include additives to prevent or diminish the transmission of ultraviolet light such as ultraviolet light absorbers (“UVAs”) and/or hindered amine light stabilizers (“HALS”) which can be used in combination with anti-oxidants, for example, phenolic antioxidants.
It is known in the art that polyurethanes can be stabilized against NOx and photodegradation through a combination of aminoplast, UVAS, and phenolic antioxidant. The effect of the combination is greater than the sum of the contributions of the individual components. The addition of the aminoplast can improve the effectiveness of the UVAs and antioxidant.
Electrodepositable primer compositions containing an aqueous dispersion of an epoxy-based ionic resin and an antioxidant additive comprising a combination of a phenolic antioxidant and a sulfur-containing antioxidant are also described in the art. Such additives are disclosed as providing reduced overbake yellowing of the subsequently applied topcoats, as well as preventing intercoat delamination of these topcoats upon exterior exposure.
A combination of a hindered phenol and an amide in a polyurethane is also known in the art. Such a combination is described as being effective against thermal and NOx induced discoloration in such polyurethane compositions.
It has been shown in the prior art that the use of spiroindane derivatives in a powder coating can help prevent oven yellowing due to the action of NOx. The invention is preferably used in conjunction with a phosphite. Phosphites are preferred to hindered phenols due to the tendency of the latter to discolor in the presence of NOx.
It has been found that many solvents and plasticizers commonly used in electrodepositable coating compositions can promote the oxidation of nitric oxide to nitrogen dioxide at typical cure temperatures, usually 90° to 200° C. The significance of this with respect to subsequent oxidation of a coating surface during thermal cure has heretofore not been previously appreciated. The discovery that electrodeposited coatings are susceptible to deleterious action by the resulting elevated levels of the nitrogen dioxide component of NOx, independent of the total NOx concentration in the oven, indicates that volatile components of the coating itself can contribute indirectly to surface oxidation and yellowing in gas-fired ovens.
The aforementioned prior art addresses overbake yellowing by protecting the film from the effects of mixtures of nitrogen oxides, through the use of nonvolatile additives that will protect the coating from the oxidizing atmosphere during cure. Accordingly, there remains a need in the coatings industry for a method to reduce the oxidative environment within a curing oven, in particular an environment comprising nitrogen oxides (NOx). A unique approach to this problem would be to develop an electrodepositable coating composition that can mediate the level of nitrogen dioxide formed in a curing oven during the curing step.